Metal Atom Vapor Chemistry: A Field Awaits Its Breakthrough

A chemical technique that generated tremendous excitement in the mid-1980s--though the hoopla has since faded somewhat in the United States--may be undergoing a renaissance in the international arena. The technique, metal atom vapor chemistry, generates single atoms of metal, which behave quite differently from their more familiar bulk metal forms--and reacting these metal atoms may someday generate valuable new materials. Before that can happen, however, problems in adapting the process to a production scale must be resolved. Researchers in the former Soviet Union and in other countries have recently been exploring the technology's potential. Despite the fact that vaporizing metal atoms is messy and time- consuming, a number of U.S. companies are working with the method as well. Some chemists and materials scientists are using it to produce catalysts for difficult chemical reactions and synthesize new compounds. Other scientists are looking at the technique as a way to produce novel materials with a variety of applications that include refining petrochemical products and improving information storage and processing. In metal atom vaporization, metal is heated in a vacuum and condensed on the cooled walls of the reactor in which the process takes place. As the metal atoms are deposited, solvent or reactant molecules are also condensed and deposited on the reactor walls--which have been chilled by immersion in liquid nitrogen. When what is now a frozen mixture of metal atoms and solvent or reactants warms, compounds form, some of which cannot be prepared by any other known synthetic technique. Surrounding a metal atom with a solvent blocks its natural tendency to agglomerate to form bulk metal. Explains National Academy of Sciences member Philip Skell, a professor emeritus at Pennsylvania State University, "Metals are very strongly bound already. When you vaporize them, you get the free atoms, which are quite different in chemical properties from what they were before." Skell pioneered the technique in the early 1970s. Intriguing Discoveries Intriguing new compounds and materials produced by metal atom technology so far have remained in the research laboratory, because scaling up the process to produce large quantities of material has proved difficult or impossible. Some discoveries had patents slapped on them, and were never heard from again. But should the long-sought scale-up problem be conquered for a valuable product of metal atom vapor chemistry, the ability to build a metallic compound atom-by-atom may just take off, for it has yielded some exciting materials. For example, the technique produces colloidal metal solutions, in which small particles of metal are suspended by trapping the metal atoms in a solvent at low temperature, and then warming them. Silver suspended in this way could have applications in the photography industry. And while colloidal solutions of metals in water have been known for centuries, the formation of metal colloids in organic solvents such as acetone greatly expands the range of chemistry possible. The resulting colloidal particles can then be used to make metal films. "Gold and platinum are used in this way, on irregularly shaped objects or plastics--areas on which you cannot normally make direct deposit of metal vapor," says Kenneth Klabunde, a professor of chemistry at Kansas State University. By contrast, he notes, conventional metal deposition, when possible, involves a tedious series of chemical reactions. Metal atom vapor chemistry can also be used to combine two metals, a procedure developed by Klabunde and dubbed SMAD, for solvated metal atom dispersion. In this process, two metals vaporized with a solvent are uniformly deposited on a support, such as alumina or silica. Klabunde has used SMAD to create bimetallic catalysts that have properties different from their component metals. A manganese-cobalt pair, for example, is used in the hydrogenation of alkenes, a reaction important in the food, petrochemical, and pharmaceutical industries, and a platinum-tin bimetallic catalyst converts crude oil to high- octane fuel. Incorporating iron with another metal in SMAD can create new types of magnetic particles, says Klabunde, who has developed a bimetallic cluster composed of iron encased in magnesium. "It is a way to protect the iron cluster, forming an interesting small magnetic particle that is stable to the environment," he explains. This particle could have potential applications in information storage and retrieval technologies, or as a chemical catalyst. Trying to create bimetallic clusters conventionally would entail reducing--adding hydrogen to--two metal salts simultaneously. This generally does not work, Klabunde says, because the metals do not separate. Metal atom vapor chemistry has allowed organometallic chemists to synthesize some compounds that were predicted to exist, but couldn't be made by other means. "Some of the syntheses we did could be done no other way," says Skell. But new and interesting compounds were few and far between, and were of interest chiefly to chemists conducting basic research, says Steven Ittel, research supervisor at E.I. Du Pont de Nemours and Co. in Wilmington, Del. The field of metal atom vapor chemistry traces its roots to a half-page description by Peter Timms, a chemistry professor at the University of Bristol, England (Chemical Communications, 1525, 1968). He reacted vaporized copper with gaseous boron trichloride, condensed the reactants on cold walls, and collected a product, diboron tetrachloride. This accomplishment was significant because gaseous copper had never before been used as a reagent in a chemical synthesis. A year later, Timms reported his expanded repertoire of other transition metals vaporized with various organic and inorganic compounds to yield interesting single-atom metal complexes (Chemical Communications, 1033, 1969). Soon after, Skell and J.J. Havel of Pennsylvania State University reported a similar approach to coaxing platinum--described as "the most inert of the metals"--to atomize and join co-vaporized substances (Journal of the American Chemical Society, 93:6687, 1971). Their platinum work was inspired by their 1965 experiments treating carbon from graphite much the same way. "I started with carbon, then branched out to metals. We asked basic questions-- how do these atoms react?" recalls Skell, who quickly realized the potential of his technique to reveal chemistry at the atomic level of virtually any element. A Flurry Of Research Work on metal atom vapor chemistry seemed to reach a crescendo of activity in 1985 and 1986, when several companies were investigating its potential. Although, according to Skell, only 15 to 20 researchers are still pursuing it in the U.S., the technology has left its mark on several areas. Initially, says Klabunde, the field of metal vapor chemistry "had quite an impact on organometallic synthesis. By 1980, it seemed to switch direction to materials science. In recent years, it has made some significant contributions to heterogeneous catalysis." A heterogeneous catalyst is one that is in a different state of matter from the reactants, such as a solid catalyzing gaseous or liquid reactants. Metal atom vaporization has been used to create heterogeneous catalysts such as a metal fixed to an inorganic support. Adds Geoffrey Ozin, professor of chemistry at the University of Toronto, "The field in many ways was responsible for all sorts of developments, such as metal cluster chemistry." Metal cluster chemistry has been tremendously important in understanding basic chemical mechanisms, and has played a significant role in catalysis for chemical industrial processes. Those who have abandoned metal atom vapor chemistry--and even those hanging on--are quick to point out its drawbacks. "It is difficult to do. You must work under vacuum, at high temperature, then at low temperature, with fancy equipment, and there's a lot to monkey around with," says Ozin, whose company, Torrovap Industries Inc. of Markham, Ontario, once sold metal atom reactors (see accompanying story). Adds Klabunde, "It looks easier to do than it is. You have to know how to handle and characterize the product." And that product may not be so simple to recover, says Steve Tetrick, a staff chemist at General Electric Co.'s Research and Development Center in Schenectady, N.Y. Tetrick uses the technique in his research, but points out its notoriously poor selectivity. "The metal atoms are so reactive that you get an array of different products. Plus, the vaporized metal goes everywhere in the reaction vessel--it's a mess," he says. Some objections to the technique even take on a philosophical air. "There was a negative feeling among the organometallic community that if you can do it with metal vapor chemistry, then you can do it conventionally," recalls Ozin. "They asked, `Is it really different if a compound is built from the atoms up?' " With such difficulties inherent in the technology, it isn't surprising that corporate interest, at least in the U.S., has dropped in recent years. A July 16, 1986, story in Chemical Week entitled "High hopes for metal vapor chemistry" dropped the names of various corporate giants with high stakes in the technology-- among them, Du Pont; Exxon Corporate Research Laboratories in Annandale, N.J.; the Amoco Research Center in Naperville, Ill.; 3M Co. in St. Paul, Minn.; and AT&T Bell Laboratories in Murray Hill, N.J. And while many of these companies are still pursuing the technology, few are touting it as they once did. Nowadays Exxon's John Bradley, whose work was described extensively in the Chemical Week piece championing the technology, will say only that he uses it "as a tool in my research." And although the 1986 Chemical Week article dubs 3M "one of the biggest players in metal vapor chemistry," 3M today claims not to have sunk much into the venture. "We haven't been a major player, and our investment in it was not large," says a spokesperson for the company. "We are not doing metal atom chemistry anymore, but we are still doing work in metal particle chemistry. Metal atom chemistry was difficult in scaling up and in the instability of the chemistry." Others disagree with the company's characterization of its role in the fledgling industry, however. "3M supported my work for five years," recalls Geoffrey Ozin. "They went so far as to do pilot plants, scale-ups, in numerous areas. There is a broad range of 3M patents on encapsulated metal clusters in liquid polymers, zeolites [a sponge-like mineral or synthetic material whose nanometer-sized pores permit select catalytic production of a large variety of organic compounds], and carbon supports--all sorts of neat materials." Adds GE's Tetrick: "3M went the whole nine yards, even using an electron gun, which is expensive." Du Pont's Ittel harbors little hope for the future of the technology. "I'm out of that business," he says. "It didn't really go anywhere, mainly because the few things people could do [with the technology], we discovered other, more conventional ways to do it." Some compounds made by the method appeared novel at first, but soon lost their apparent uniqueness. "For example," he says, "we put down iron atoms on alumina, which was an extremely active catalyst in the first few minutes. But a short time after, the activity declined to the activity you would get for a catalyst of the same composition made by wet chemical techniques. It was fascinating, but it didn't hold up." Swan Song In some ways, Geoffrey Ozin's experience reflects the development of the field. His article "New materials from metal vapor chemistry" (CHEMTECH, August 1985, page 488), detailed application after application, such as photography, printing, catalysis, and information storage and processing. "The CHEMTECH paper was my swan song," he says now. He had written the paper to be delivered at the 1985 American Chemical Society's Leo Friend award symposium honoring the technology. "The turnout was so abysmal that I thought, if this represents the field, there's no future in it for me," says Ozin, who, at the beginning, was "tremendously optimistic about the field." He has since moved successfully into the area of nanoporous materials. "A lot of what I learned in metal vapor chemistry helped me move into my present work encapsulating semiconductor clusters in nanoporous materials, which is a natural extension of metal vapor chemistry," he says. But metal atom vapor chemistry may be a technological phoenix, arising anew from its domestic ashes in a more international arena. Ken Klabunde recently returned from a meeting entitled "Chemistry at Low Temperatures," at Moscow State University, where he and a researcher each from Germany, France, and Britain- -all working on metal atom vapor chemistry--were the only foreigners. After hearing the talks, Klabunde predicts that researchers in Japan and the former Soviet Union will pick up where U.S. industry has left off. "The Russians and the Japanese are very interested," he notes. "I'm afraid the Japanese are getting into it, and they will eventually commercialize it--the Soviets, too. "The Russians are a little behind the times in one sense, in that they're still doing things similar to what we did in the U.S. years ago," adds Klabunde. "But on the other hand, they are very persistent, and have lots of people involved, and have made inroads that we haven't." He points out, for example, that Russian scientists have recently produced a compound using metal atom vaporization that he had predicted years ago to exist. Waiting In The Wings Despite the current chilly climate for metal atom vapor chemistry in the U.S., many concur that it is waiting in the wings for a breakthrough, perhaps a new colloid or catalyst that has valuable properties and can survive the all-important test of scale-up. Ken Klabunde is optimistic. "I think eventually it will come to fruition," he says. "There is a whole periodic table of elements as a playground, and we can work with atoms that are reactive species to create new compounds, or build small atomic clusters for use as catalysts, films, magnetic materials. "There is a lot of potential. It is just waiting for that one person to make that one brilliant discovery so someone will come forward and commercialize it." Ricki Lewis teaches biology at the State University of New York, Albany, and is the author of Life, a college biology text (Dubuque, Iowa, Wm. C. Brown Publishing, 1992).

Metal Atom Vapor Chemistry: A Field Awaits Its Breakthrough

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